The invention relates to a method for checking a subscriber line, preferably with an analog/digital subscriber line circuit for an analog telephone, and to device for checking a subscriber line.
In this context, a telecommunication station is connected to the subscriber line (known as SLCA=Subscriber Line Circuit A) by means of two electrical lines (wires). The subscriber line has a subscriber line circuit and an (analog/digital) encoder/decoder which allow connection to a digital telecommunication network. In the event of complaints from subscribers or to check assured properties, it is necessary to test the proper operation of the subscriber line circuit. One of these tests fundamentally involves measuring a degree of symmetry at inputs of the subscriber line circuit, for example because a common mode interference signal there in the event of any asymmetry adversely affects the signal quality of the subscriber line circuit.
For the purpose of ascertaining this asymmetry, a first exemplary embodiment is shown in FIG. 1. Connections of a subscriber station, i.e. inputs (wires a, b) of a subscriber line circuit SLCA with a first circuit SLIC (Subscriber Line Integrated Circuit) which supplies the wires and an encoder/decoder CODEC (Coder-Decoder), have an external meter EM connected to them which is used to generate a tone Uext and to simultaneously feed it via outputs of the meter EM into a respective wire a, b via a resistance RVa, RVb (e.g. 300 Ohms) in DC-decoupled fashion. This tone forms a common mode signal and, following reflection at the subscriber line circuit, is measured as a voltage Uqu between the outputs of the meter EM. The measured voltage Uqh provides a measure of the asymmetry in the subscriber line circuit. However, this method requires the use of an external meter.
The invention is based on the object of ascertaining a degree of asymmetry between two connections on a subscriber communication station without using an external meter.
In this context, a method for checking a subscriber line with a first and a second electrical line which are each connected to an encoder/decoder via a subscriber line circuit is first of all presented. The fact that:
A)                the first line is connected to an electrical ground,        a resistance of the second line is increased to a high value (or such that no current flows in the open line),        the encoder/decoder outputs an audio signal into the first line which is furthermore reflected in the first line,        a first frequency-selective signal is measured therefrom at the point at which the encoder/decoder is connected to the first line,        the resistance of the second line is reset and the first line is connected to its original connection,        
B)                the second line is connected to the electrical ground,        a resistance of the first line is increased to a high value,        the encoder/decoder outputs a further audio signal into the second line which is likewise reflected in the second line,        a second frequency-selective signal is measured therefrom at the point at which the encoder/decoder is connected to the second line,        
C)                an amplitude difference between the first and second frequency-selective signals is ascertained        